1. Field of the Invention
The present invention relates to an optical waveguide used for optical information processing and applied optic measurement, which utilize a coherent light source, a wavelength conversion device employing such an optical waveguide, and methods for fabricating the optical waveguide and the wavelength conversion device.
2. Description of the Related Art
An optical waveguide enjoys general uses in a wide range of fields such as communications, optical information processing and optic measurement as an optical wavelength control technique. If the optical waveguide is applied to an optical wavelength conversion device, a small-sized short-wavelength light source capable of converting a wavelength of a laser beam (a fundamental wave) emitted from a semiconductor laser by using the optical wavelength conversion device so as to obtain light having a short wavelength (a second-harmonic wave) is realized. Therefore, active studies have been made on such an optical waveguide.
A buried type optical waveguide has been generally utilized as a conventional optical waveguide. FIGS. 1A to 1D are cross-sectional views illustrating a conventional method for fabricating the buried type optical waveguide through a proton exchange treatment for exchanging proton ions (+H) and a thermal treatment.
More specifically, as shown in FIG. 1A, a Ta (tantalum) layer 12 having a strip-like opening is formed on a surface (+C face) of a LiTaO.sub.3 substrate 11. The Ta layer 12 serves to mask the surface of the LiTaO.sub.2 substrate 11 except for a predetermined region.
Next, the LiTaO.sub.3 substrate 11 masked with the Ta layer 12 is thermally treated with pyrophosphoric acid at a temperature between about 220.degree. C. and about 300.degree. C. As a result, a region 13 which is subjected to a proton exchange treatment (hereinafter, also referred to as "proton-exchanged region") is formed in the vicinity of the surface of the exposed part of the LiTaO.sub.3 substrate 11 as shown in FIG. 1B.
Subsequently, the Ta layer 12 is immersed into a mixed solution of hydrofluoric acid and nitric acid at a ratio of 1:2, thereby removing the Ta layer 12 as shown in FIG. 1C.
Thereafter, the LiTaO.sub.3 substrate 11 is annealed so as to form an annealed proton-exchanged layer 14 as shown in FIG. 1D. The resulting formed annealed proton-exchanged layer 14 functions as a buried type optical waveguide.
On the other hand, an optical wavelength conversion device employing a ridge type optical waveguide structure in order to enhance a light confinement efficiency is disclosed, for example, in Japanese Laid-Open Patent Publication No. 1-238631.
FIG. 2A shows an example of the structure of a conventional optical wavelength conversion device including such a ridge type optical waveguide 22. As can be appreciated from FIG. 2A, the surface of the optical waveguide 22 formed on a LiNbO.sub.3 substrate 21 is processed into a ridge shape. More specifically, as shown in FIG. 2B which is a cross-sectional view taken along a line 2B--2B of FIG. 2A, a portion 22a through which light is guided (that is, a ridge portion) has a thickness d greater than a thickness h of a side portion 22b. As a result of providing a ridge portion serving as the waveguide portion 22a of light for the optical waveguide 22, lateral light confinement is enhanced so as to increase the power density of a fundamental wave within the optical waveguide 22, thereby realizing the improvement of a conversion efficiency from a fundamental wave into a second-harmonic wave.
Furthermore, regarding the structure of the optical wavelength conversion device shown in FIG. 2A, a light input portion 23 for the LiNbO.sub.3 substrate 21 is configured so that a thickness of the optical waveguide portion 22 is thicker than that of the other part in the vicinity of an end face 24 from which a fundamental wave P1 enters. As shown in FIG. 2C which is a cross-sectional view taken along a line 2C--2C shown in FIG. 2A, after the fundamental wave P1 entering the LiNbO.sub.3 substrate 21 from the end face 24 through the light input portion 23 is subjected to a wavelength conversion into a second-harmonic wave P2, the second-harmonic wave P2 goes out from a light output portion 25 toward the outside of the LiNbO.sub.3 substrate 21.
Furthermore, for example, Japanese Laid-Open Patent Publication No. 61-94031 discloses an optical wavelength conversion device utilizing a strip-loaded optical waveguide. FIG. 3 shows an example of the structure of such an optical wavelength conversion device.
More specifically, an optical waveguide 32 is formed on a LiNbO.sub.3 substrate 31 by proton exchange. On the optical waveguide 32, a stripe-shaped cladding layer (strip-loaded layer) 33 made of SiO.sub.2, having a lower refractive index than that of the optical waveguide 32, is formed. Since the resulting structured strip-loaded optical waveguide has a low loss, an optical wavelength conversion device for converting the fundamental wave P1 into the second-harmonic wave P2 with high efficiency is realized.
In addition, the following method for forming a ridge type waveguide on a LiNbO.sub.3 substrate has been reported.
Since LiNbO.sub.3 is a mechanically and chemically stable material and is thus hardly etched, the etching selectivity to a resist is small. Therefore, it is generally difficult to deeply etch the surface of a LiNbO.sub.3 substrate. However, an etching rate for an LiNbO.sub.3 substrate, which is subjected to a proton exchange treatment, is increased to be several times that of an untreated substrate. By utilizing this phenomenon, a method for fabricating a ridge type optical waveguide as follows has been proposed. A LiNbO.sub.3 substrate serving as a C plate is thermally treated in an appropriate acid so as to form a proton-exchanged layer thereon. Then, a stripe-shaped Ti protection mask layer is formed on the resulting formed proton-exchanged layer by photolithography. Thereafter, an unmasked region is etched by ECR etching. Then, the Ti protection mask layer is removed. Both end faces of the optical waveguide are optically polished, thereby forming a light input portion and a light output portion.
FIG. 4 shows another structure of an optical wavelength conversion device utilizing a buried type optical waveguide. A proton-exchanged optical waveguide 42 is formed on an LiNbO.sub.3 substrate 41. On the proton-exchanged optical waveguide 42, a cladding layer (strip-loaded layer) 43 made of TiO.sub.2 having a refractive index higher than that of the optical waveguide 42 is formed. A plurality of domain-inverted layers 44 are formed so as to perpendicularly cross the optical waveguide 42 in a periodic manner. A fundamental wave P1 entering the structure from the light input portion 45 overlaps the domain-inverted layers 44 while propagating through the optical waveguide 42. The fundamental wave P1 is converted into a second-harmonic wave P2, and the second-harmonic wave P2 then goes out from the light output portion 46.
FIGS. 5A and 5B schematically show the overlapping relationship between a guide mode of the optical waveguide 42 and the domain inverted layers 44. FIG. 5A shows the case where the cladding layer (strip-loaded layer) 43 is not provided, while FIG. 5B shows the case where the cladding layer 43 is provided. In each of FIGS. 5A and 5B, a cross-sectional structure of the optical wavelength conversion device is shown on the left, and an electric field distribution of the propagating light in a depth direction of the cross-section of the optical waveguide is shown on the right.
In the configuration shown in FIG. 5A which does not have a cladding layer, merely about half (a hatched area of the electric field distribution shown in FIG. 5A on the right) of propagating light (fundamental wave P1) overlaps the domain-inverted layers 44 to be converted into a harmonic wave. Therefore, the resultant harmonic wave output is not as high as expected. On the other hand, in the configuration shown in FIG. 5B having the cladding layer 43 with a high refractive index on the optical waveguide 42, a major part (a hatched area of the electric field distribution shown in FIG. 5B on the right) of propagating light (fundamental wave P1) overlaps the domain-inverted layers 44. Thus, the optical wavelength conversion device for converting the fundamental wave P1 into the second-harmonic wave P2 at a high efficiency is realized.
When the optical waveguide is used in combination with the optical wavelength conversion device, the degree of overlap between waveguide modes (the fundamental wave and the second-harmonic wave) is an important factor for realizing an optical wavelength conversion device with high efficiency. More specifically, as the degree of overlap between the electric field distribution of the fundamental wave and that of the second-harmonic wave becomes greater, a conversion efficiency of the optical wavelength conversion device increases. In order to increase the degree of overlap between the electric field distribution of the fundamental wave and that of the second-harmonic wave so as to realize an optical wavelength conversion device with high efficiency, a refractive index distribution of the optical waveguide has a stepwise pattern.
With the conventional method for fabricating the buried type optical waveguide through a proton exchange treatment and a thermal treatment as described above, however, the resultant optical waveguide has a graded refractive index distribution. That is, the refractive index becomes maximum in the vicinity of the surface and gradually decreases in a depth direction depending on a thermal diffusion state of the proton. With such a refractive index distribution, it is difficult to realize an optical wavelength conversion device with high efficiency because the electric field distribution of the guided fundamental wave greatly differs from that of the second-harmonic wave. By this conventional method, a refractive index distribution of the optical waveguide cannot be arbitrarily controlled.
Moreover, in a conventional buried type optical waveguide, optical damage tends to occur due to light leakage in the peripheral region of the optical waveguide. Thus, it is difficult to guide light with high power density so as to generate a second-harmonic wave with high output.
In the structure having a cladding layer, a high efficiency is intended to be realized by enhancing the light confinement of the optical waveguide. More specifically, by moving a mode distribution (a fundamental mode) of guided light propagating through the optical waveguide toward the vicinity of the surface by cladding layer, the degree of overlap between domain-inverted regions and a fundamental wave of a fundamental mode or a second-harmonic wave of a fundamental mode is intended to be improved.
Although in the conventional structure, the degree of overlap between the domain-inverted regions and the waveguide mode is increased, the degree of overlap between the fundamental wave and the second-harmonic wave, which has the greatest effect on a conversion efficiency, is not increased. Therefore, an improvement of the conversion efficiency is limited. This results from the limitation of an increase in the degree of overlap between the waveguide modes of the fundamental wave and the harmonic wave because the distributions of the waveguide modes in the optical waveguide greatly differ from each other due to a difference in wavelength.
Furthermore, since the area in which the fundamental wave and the second-harmonic wave does not overlap is large, a second-harmonic wave output may be unstable due to the occurrence of an optical damage.
On the other hand, in a conventional optical wavelength conversion device using a ridge type optical waveguide, the power density is increased by a light confinement effect of the optical waveguide to improve a conversion efficiency. However, an increase in the confinement effect by the ridge type optical waveguide for a fundamental wave is achieved solely in a lateral direction. The confinement effect is not improved in a depth direction. Thus, the degree of overlap between the fundamental wave and the second-harmonic wave, which has the greatest effect on a conversion efficiency, is not achieved with the ridge type structure (particularly, in a depth direction). Therefore, an improvement of the conversion efficiency is limited.